1. Field of the Invention
This invention relates to an analog/digital converter, used for a document reader such as a facsimile or a scanner, which converts the analog output signal of a photoelectric converting element into digital values.
2. Description of the Related Art
CCD (charge-coupled device) sensors are now widely used as photoelectric converting elements in document readers such as facsimiles or image scanners. CCD sensors of this type have undergone various improvements, so that even large-scale CCD sensors are now available.
With the increasing need for high-speed operation, high-quality picture, and coloration in such document readers, the signal processing of CCD sensor outputs is becoming more sophisticated. To achieve such sophisticated signal processing, special large-scale integrated circuits (LSIs) have been developed.
In conventional LSIs, however, the input level of analog signals to the LSIs must be adjusted using externally added variable resistors. The input voltage range to the A/D converter, which converts the CCD sensor output signal into digital values, must be held constant. Such adjustment has been done manually, which resulted in longer adjustment time for each unit and poor accuracy.
As noted above, conventional LSIs had the drawbacks of requiring manual adjustment of the analog signal input to LSIs and lacking accuracy.